Root rot/rhododendron wilt fungus is possible, and if that's the case there isn't much you can do for the shrub.

Other possibilities - were you careful to plant it no deeper than it was growing in the nursery container so the surface roots would be able to function? Loosen the rootball well before transplanting?

If you didn't buy potting medium made specifically for rhododendron/azalea, the container mix for rhododendrons is pretty specific...50 - 75% organic matter, combination being made up of pre-wetted peat, compost, sand, ground bark (sand or ground bark of at least 25%).

Even then, if you live where summer rainfall might coincide with hot humid weather, containerizing these is iffy and a generous amoung of perlite or bark chips should be added to the above materials...placing emphasis on aeration. Another recommendation is to use oversized pots to improve the unfavorable heat/moisture conditions often found in smaller containers.

Why are you planting in the container?